There is no in depth information to know yet. One would assume you will be seeing that kind of information coming out this month and next month.
No in depth information about AZs and the like at end game, rest assured that we'll hear something in the coming weeks.
They've planned an awful lot of PvE content and stressed the benefits of grouping together, for some obvious yet effective reasons.
I guess many MMO (like WoW) add content periodically and nobody asks why they do not plan - it just happens. I guess here is the same. The only option that could be more explained (or planned if not done yet) is how they imagine progress in years of game play. Uncapped level games has this actually simply responded - with every extension level cap is changed and all items need to be replaced to be on top and new content allows to get these items.
If ESO is planned with 50 level cap then there is obvious question (doubt / afraid) how is it planned to further build / extend player character.
Horizontally at first, with new content and skill lines added, eventually it would have to go vertical though, and level caps raised.